The present invention relates to transfer cases for automotive vehicles having a plurality of pairs of traction axles. More particularly, the invention relates to a transfer case having an input shaft driven by a prime mover of an automotive vehicle through a main transmission and a plurality of output shafts operatively connected to a plurality of pairs of traction axles, respectively, to drive them.
A four-wheel drive automotive vehicle having such a drive power path as above presents the following problems when it is driven through a paved road with a four-wheel drive.
(1) On cornering (i.e., turning), the front wheels tend to slide, causing a great deal of effort, on the part of a driver, for manipulating a steering wheel and a great deal of front tire wear, because the front wheels must overrun the rear wheels to run through a curve having radius of curvature longer than a curve which the rear wheels run through.
(2) In the case that there are variations in effective tire diameters due to tire overinflation and/or tire wear and/or imbalance of load on the vehicle body, such wheel or wheels having a smaller diameter will act as a brake, making it difficult to keep the vehicle moving in a straight line.
(3) The braking phenomenon as described above will also impose severe stress on the instruments between the engine and the traction wheels.
To eliminate these problems, it is proposed to provide, in addition to a front differential mechanism between front wheels and a rear differential mechanism between rear wheels, a third differential mechanism between the front and rear differential mechanisms to permit the front wheels to outrun the rear wheels on cornering and those wheels which have relatively small effective diameter to outrun the remaining wheels.
Since it is not desirable to permit differential action between the front wheels and the rear wheels if one or some wheels slip when the vehicle is driven through rough terrain, such as sand, mud or snow, the third differential mechanism must be provided with a mechanism to limit its differential action for facilitating moving the vehicle through or out of sand, mud or snow. Since, under some circumstances, limitation in some degree of the differential action is not sufficient for facilitating extraction of the vehicle if logged in sand, mud or snow, a lock-up mechanism must be installed to prevent the differential action of the third differential mechanism.
Besides, it is necessary to install, in addition to a main transmission for high-speed low torque driving through a paved road, an auxiliary transmission for lowspeed high-torque off-road driving, because excessively high load would be imposed to the engine if the vehicle is driven at low-speed high-torque with the main transmission.
It will now be understood that for off-road driving without imparing on-road driving, a third differential between the front and rear axles with a lock-up mechanism and another transmission for off-road driving in addition to a main transmission for on-road driving need to be installed in a vehicle.